Cancer development, tumor formation, progression, invasion and metastasis all involve selective mRNA translation but much of the understanding by which oncogenic translation occurs remains to be characterized. We recently discovered a new mechanism of cap-dependent mRNA translation that established a new paradigm for translational regulation, in that it is cap-dependent but does not involve the canonical pre-initiation machinery. We showed that the canonical eIF4E/eIF4GIfactors are not required for approximately 20% of mRNAs that use the translation initiation complex known as DAP5/eIF3d, which utilizes the cap binding activity of translation initiation factor eIF3d rather than eIF4E and eIF4GI homolog, DAP5. We found that DAP5/eIF3d is essential for breast cancer metastasis in animal models and strongly associated with human breast cancer metastasis. Here we propose to study the role of DAP5/eIF3d-mediated mRNA translation in breast cancer development and metastasis. We will determine the roles of DAP5/eIF3d-mediated mRNA translation in tumor development, invasion and metastasis, using syngeneic mouse mammary cancer models, human xenotransplant breast cancer cell line models and a heterogeneous patient derived xenograft model, the latter two models carried out in immune impaired mice. We will characterize the requirement for DAP5/eIF3d versus eIF4GI/eIF4E initiation of translation on cancer cell epithelial to mesenchymal transition (EMT), stromal tissue invasion, vascular intravasation, extravasation, migration, prevention of apoptosis resulting from cell detachment and migration, and colonization. Other studies will characterize by genome-wide trancriptomic and translatomic analyses the mRNAs selectively translated by DAP5/eIF3d and required for progression to metastasis, compared to mRNAs translated by eIF4E/eIF4GI in animal models. These studies will further our understanding of the role of DAP5- selective mRNA translation in driving specific oncogenic pathways involved in metastatic tumor progression. We will also test the importance of specific DAP5 mRNA translation targets in DAP5-mediated metastatic tumor progression. Finally, studies will be carried out to understand the mechanism(s) for DAP5/eIF3d-mediated selective mRNA translation. Studies will focus on the role of a specific form of mRNA modification by methylation known as m6A because we have found strong preliminary data in support of its importance.